为什么我的vb.net htmlelementCollection代码在线程中使用时会抛弃错误

Question

我已经在vb.net中编写了一些代码，这些代码'抓取'图像的src/url基于其'alt'属性。它可以在某些站点上完美工作，但是在其他站点上，它需要线程 - 等待直到加载图像，因为图像在发生记录结束时没有加载。

。

我的问题是，当我的代码用于线程时，我会收到invalidcastexception错误。

这是我的代码，发生错误的地方：

Private Sub WB1_DocumentCompleted(sender As Object, e As WebBrowserDocumentCompletedEventArgs) Handles WB1.DocumentCompleted
    urlBox.Text = e.Url.ToString()
    If urlBox.Text = "URL_GOES_HERE"
        Dim success As Integer = site.fill() 'calls another function, returns 1 on success
        If success = 1 Then
            Dim captchaThread As New Thread(New ThreadStart(AddressOf getCaptchaURL))
            captchaThread.Start()
        End If
    End If
End Sub
Public Function getCaptchaURL()
    Thread.Sleep(5000) 'Tell thread to sleep so images can load
    Dim URL As String
    Dim images As HtmlElementCollection = WB1.Document.GetElementsByTagName("img") 'This is where I get the 'invalidcastexception' error
    For Each element As HtmlElement In images
        Dim source As String = element.GetAttribute("alt").ToString
        If source = "CAPTCHA Image" Then
            URL = element.GetAttribute("src")
            MessageBox.Show(URL) 'Show URL to check the source has been grabbed
        End If
    Next
    Return URL 'Return URL for further functions
End Function

so，为了澄清，此代码：Dim images As HtmlElementCollection = WB1.Document.GetElementsByTagName("img")在与线程一起使用时给我一个错误，但在线程中不使用时不使用。

Answer 1

行

For Each element As HtmlElement In images

需要重新编写以避免遇到铸造类型的问题。

尝试以下内容：

For Each element In images
   If TypeOf(element) Is HtmlElement Then
         'the rest of your code goes here
   End If
Next